The present invention relates to a vehicle control system for controlling the stability and control characteristics of a wheeled vehicle such as an automobile, and more specifically to a vehicle control system designed to maintain a steady state side slip angle of a center of gravity of the vehicle constant regardless of changes in vehicle speed.
The possibility of improvement in steering characteristics of a conventional two wheel steer vehicle (in which only front wheels are steered by angular displacement of a steering wheel) is limited because of the very nature of the two wheel steer vehicle.
A conventional example of a control system for a four wheel steer vehicle is disclosed in `Gakujutsu Koenkai Zensatsushu 842058` (pages 307-310) published by Society of Automotive Engineers of Japan in 1984. The control system of this example is arranged to maintain a steady state side slip angle .beta. at zero to eliminate a phase delay of a vehicle lateral acceleration by controlling a rear wheel steer angle. However, in a high vehicle speed range (over 100 km/h, for example), this system cannot attain the intended result that .beta.=0 without increasing the rear steer angle so much that the yaw rate gain of the vehicle decreases rapidly with increase in vehicle speed because this system is so devised as to control the side slip angle by manipulating the rear wheel angle only. Therefore, the handling characteristics obtainable by this system are unsatisfactory in that a great amount of steering is required for changing the direction of the vehicle as, for example, in changing lanes. Furthermore, this conventional control system cannot yield a stable vehicle response in a transient state which results from an input to the steering wheel until a steady state condition is reached.